Cross-Cultural Validity and Reliability of the Questionnaire on Back-Health-Related Postural Habits During Daily Activities in the Polish Young Adolescent Population

Background/Objectives: This study aimed to conduct the cross-cultural adaptation and assess the reliability of a validated measurement instrument, the Back-Health Related Postural Habits in Daily Activities (BEHALVES). Methods: Following a standardized methodological framework for the cross-cultural adaptation of health-related instruments, the BEHALVES questionnaire was translated and tested in 104 primary education students (mean age 13.8 ± 0.74 years; 49.1% girls) at two different time points with a 1-week interval between each test. The analysis was expressed as test–retest reliability according to the differences observed between the readings (T1–T2, α = Cronbach’s alpha), the standard deviation of the differences (SD), the intraclass correlation coefficient (ICC), 95% confidence intervals for the intraclass correlation coefficient, standard error of measurement (SEM), coefficient of repeatability (CR), and minimal detectable change (MDC). Results: Internal consistency results for the joint analysis of items grouped into categories were good (α = 0.72). Lying down was the worst-performing category in this study. The ICC was excellent for the total score (0.96) and all categories (0.90–0.99). Regression analysis between the mean of the two measures and their difference indicated that there was a significant difference (F1,103 = 0.19, p < 0.001; beta coefficient = −0.15, p < 0.001; R2 = 0.16). Conclusions: The BEHALVES questionnaire is valid and reliable for the Polish youth population, providing similar results to the original Spanish version. The continued development of reliable measurement instruments will enhance research in education, public health, and clinical practice, ultimately leading to improved strategies for preventing postural issues in young populations.